Climate change protesters accused of hijacking a power station coal train managed to address a jury on political issues today, despite repeated warnings from the judge that the jury was only concerned with whether they had stopped and boarded the train – and not with their reasons for doing so.

Judge Spencer had repeatedly told Leeds University lecturer Paul Chatterton, who is leading the defence of 22 activists, that the court was not concerned with their motives for allegedly ambushing the train on its way to Drax in North Yorkshire, Europe's largest coal-fired power station.

But after almost an hour's adjournment he allowed Chatterton and a second protester, 26-year-old film-maker Alison Stratford, wider scope before finally intervening to cut them short.

Chatterton, who has lectured on geography for 11 years, said that he had acted because of "passion and terror at the implications of coal-burning power stations for global warning".

He told the jury that he did not consider the train hijack to be an illegal act, because United Nations statistics suggested that the amount of carbon produced by Drax was responsible for 180 deaths a year.

He said: "The threat is deadly and it is urgent …" The judge then interrupted him again, saying: "I've let you go on – please remember the legal restraints."

Stratford was allowed to show the jury photographs of houses under water in her home town of Louth, Lincolnshire, which she said had roused her fears about climate change. She choked and had to recover in the witness box as she described how her four-year-old nephew had told her: "Don't worry, we can fix it."

"I was on the train to show him that I had done everything I could," she said. But when she got on to Arctic ice melt and polar bears, the judge again asked: "Could you talk about the train?"

The court heard that the protesters had lined up academic witnesses and a scientist from Nasa to address the jury, but this had been ruled inadmissible. Adjourning for lunch, the judge warned that he would show less patience if defendants insisted on talking about their "genuine and deeply held feelings about climate change" rather than the nuts and bolts of the train hijack.

The defendants, aged between 21 and 43, have pleaded not guilty to obstructing a railway engine contrary to the Malicious Damage Act of 1861. But Chatterton admitted as soon as he began his defence that he had been on the train and had "intended to stay on it as long as possible". Earlier he told the jury that the prosecution case, which began and ended yesterday, had been "incredibly partial" about the incident on 13 June last year.

Addressing the seven women and five men directly across the crowded courtroom at Leeds crown court, he said: "They said what went on there but did not deal with why."

Yesterday, Richard Mansell QC, prosecuting, told the jury that the defendants were "preparing a misuse of the court process to continue the protest action which they started when they boarded that train just over a year ago".